Fluoropolymer containing laminates

ABSTRACT

The multilayer film serves as a laminate. The film is a multilayered structure that, in its base form, encompasses an intermediate layer with first and second outer layer affixed to opposing sides of the intermediate layer. The first outer layer is a semi-crystalline fluoropolymer. The intermediate layer includes a polyester and the second outer layer is an olefinic polymer. The layers are bonded together in the noted order to provide the multilayer film.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationNo. 60/689,840, filed Jun. 13, 2005, herein incorporated by reference inits entirety.

TECHNICAL FIELD

This invention relates to multilayer fluoropolymer films or laminates,and methods for their manufacture that are useful as a backing film forsolar cells.

BACKGROUND

Multilayer films or laminates are constructions, which attempt to marrythe properties of dissimilar materials in order to provide an improvedperformance. Such properties include barrier resistance to elements suchas water, cut-through resistance, weathering resistance and electricalinsulation. Previous laminates have addressed many of the needs forsolar modules, but often result in a mis-balance of properties, are moreexpensive, or difficult to handle or process. In addition, the innerlayers are often not fully protected over the life of the module.

In order to improve the durability, longevity and performance ofphotovoltaic modules, backside laminates are being developed withthicker layers of barrier materials such as PET, (polyethyleneterepthalate), or resort to the use of metal foils, inorganic coatings,or multiple layers of fluoropolymers. These endeavors typical result inconstructions, which are often more expensive, and/or laminates whichare stiffer (i.e. of higher modulus), and that are more difficult toapply to the backside of solar modules. Additionally, the conventionalconstructions typically require that the completed, typicallymultilayer, construction be subjected to a heating cycle prior tolamination so that the entire construction can be successfullylaminated.

SUMMARY

Briefly, the present invention is directed to a multilayer film. Themultilayer film includes a polyester intermediate layer having first andsecond outer layers bonded to opposing sides of the intermediate layer.The first outer layer is a semi-crystalline fluoropolymer with a tensilemodulus of less than 100,000 psi. The intermediate layer has a shrinkagerate of less than 1% at 150° C. when held for about 15 minutes and asecond outer layer is an olefinic polymer.

The invention also provides a process for preparing a layered articlecomprising providing a first intermediate layer of a polyester having afirst and second surface, wherein the first layer has a shrinkage ofless than about 1% at 150° C. when held for about 15 minutes; applying afirst outer layer of semi-crystalline fluoropolymer, with a tensilemodulus of less than 100,000 psi, to the first surface; applying asecond outer layer of an olefinic polymer to the second surface; andbonding the layers to form the multilayer film through the applicationof energy, mechanical forces or both.

Other features and advantages of the invention will be apparent from thefollowing detailed description of the invention and the claims. Theabove summary of principles of the disclosure is not intended todescribe each illustrated embodiment or every implementation of thepresent disclosure. The following details more particularly exemplifycertain preferred embodiments utilizing the principles disclosed herein.

DETAILED DESCRIPTION

The present invention is a multilayer film that serves as a laminate. Ina preferred embodiment, the multilayer film provides durability,longevity and performance enhancements of photovoltaic modules when itis utilized as a backside film on the modules. The film is amultilayered structure that, in its base form, encompasses anintermediate layer with first and second outer layer affixed to opposingsides of the intermediate layer. The first outer layer is asemi-crystalline fluoropolymer with a tensile modulus of less than100,000 psi, as defined in ASTM D638. The intermediate layer includes apolyester having a shrinkage rate of less than 1% measured at 150° C.when held for about 15 minutes. The second outer layer is an olefinicpolymer. The layers are bonded together in the noted order to providethe multilayer film.

In it's various aspects, the fluoropolymer component of the first outerlayer can be selected from a variety of fluoropolymers. Suchfluoropolymers are typically copolymers of TFE or VDF with otherfluorinated or non-fluorinated monomers. Representative materialsinclude copolymers of tetrafluoroethylene-ethylene (ETFE),tetrafluoroethylene-hexafluoropropylene (FEP),tetrafluoroethylene-perfluoroalkoxyvinlyether (PFA), copolymers ofvinylidene fluoride and chlorotrifluoroethylene,tetrafluoroethylene-hexafluoropropylene-ethylene (HTE), copolymers ofvinylidene fluoride and chlorotrifluoroethylene, or a copolymer derivedfrom tetrafluoroethylene (TFE), hexafluoropropylene (HFP), andvinylidene fluoride (VDF), such as the THV series available from DyneonLLC, Oakdale, Minn.

The first outer layer possesses a tensile modulus of less than 100,000psi. The noted tensile modulus is directed to achieving desired flexuralcharacteristics in order to make the finished film structure pliable inits intended application. Additionally, the fluoropolymer outer layer ispreferably capable of providing low permeability characteristics to theconstruction in order to protect internal components of the file or ofthe preferred solar cell application.

A preferred class of fluorinated copolymers suitable as the first outerlayer are those having interpolymerized units derived fromtetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride, andoptionally a perfluoro alkyl or alkoxy vinyl ether. Preferably thesepolymers have less than about 30 weight percent (wt %) VDF, morepreferably between about 10 and about 25 wt %, of its interpolymerizedunits derived from VDF. A non-limiting example includes THV 500available from Dyneon LLC, Oakdale, Minn.

Another preferred class of materials suitable for use as the first outerlayer include various combinations of interpolymerized units of TFE andethylene along with other additional monomers such as HFP, perfluoroalkyl or alkoxy vinyl ethers (PAVE or PAOVE). An example being HTE 1510,available from Dyneon LLC, Oakdale, Minn.

The present invention contemplates that any polyester polymer capable ofbeing processed into film form may be suitable as an intermediate layer.These may include, but are not limited to, homopolymers and copolymersfrom the following families: polyesters, such as polyethyleneterephthalate (PET), and liquid crystalline polyesters.

In alternative embodiment, the intermediate may include other polymerssuch, for example: polyarylates; polyamides, such as polyamide 6,polyamide 11, polyamide 12, polyamide 46, polyamide 66, polyamide 69,polyamide 610, and polyamide 612; aromatic polyamides andpolyphthalamides; thermoplastic polyimides; polyetherimides;polycarbonates, such as the polycarbonate of bisphenol A; acrylic andmethacrylic polymers such as polymethyl methacrylate; chlorinatedpolymers, such as polyvinyl chloride and polyvinylidene chloride;polyketones, such as poly(aryl ether ether ketone) (PEEK) and thealternating copolymers of ethylene or propylene with carbon monoxide;polystyrenes of any tacticity, and ring- or chain-substitutedpolystyrenes; polyethers, such as polyphenylene oxide,poly(dimethylphenylene oxide), polyethylene oxide and polyoxymethylene;cellulosics, such as the cellulose acetates; and sulfur-containingpolymers such as polyphenylene sulfide, polysulfones, andpolyethersulfones.

A most preferred material is polyethyleneterepthalate, (PET).

In one aspect of the invention, the intermediate layer is pre-shrunkprior to formation of the multilayer film. The shrinking of thepolyester intermediate layer results in an intermediate layer that willshrink less than 1% of it's total length in either planer direction whenexposed to a temperature of 150° C. during a period of 15 minutes, inaccordance with ASTM D 2305-02. Such films are commercially available orcan be prepared by exposing the film, under minimal tension, to atemperature above it's glass transition temperature, preferable above150° C. for a period of time sufficient to pre-shrink the film. Suchthermal treatment can occur either as a post treatment or during theinitial manufacturing process used to produce the film.

Olefinic polymers useful in the composition of the invention as an outerlayer include polymers and copolymers derived from one or more olefinicmonomers of the general formula CH₂═CHR″, wherein R″ is hydrogen orC₁₋₁₈ alkyl. Examples of such olefinic monomers include propylene,ethylene, and 1-butene, with ethylene being generally preferred.Representative examples of polyolefins derived from such olefinicmonomers include polyethylene, polypropylene, polybutene-1,poly(3-methylbutene), poly(4-methylpentene) and copolymers of ethylenewith propylene, 1-butene, 1-hexene, 1-octene, 1-decene,4-methyl-1-pentene, and 1-octadecene.

The olefinic polymers may optionally comprise a copolymer derived froman olefinic monomer and one or more further comonomers that arecopolymerizable with the olefinic monomer. These comonomers can bepresent in the polyolefin in an amount in the range from about 1 to 10wt-% based on the total weight of the polyolefin. Useful such comonomersinclude, for example, vinyl ester monomers such as vinyl acetate, vinylpropionate, vinyl butyrate, vinyl chloroacetate, vinyl chloropropionate;acrylic and alpha-alkyl acrylic acid monomers, and their alkyl esters,amides, and nitriles such as acrylic acid, methacrylic acid, ethacrylicacid, methyl acrylate, ethyl acrylate, N,N-dimethyl acrylamide,methacrylamide, acrylonitrile; vinyl aryl monomers such as styrene,o-methoxystyrene, p-methoxystyrene, and vinyl naphthalene; vinyl andvinylidene halide monomers such as vinyl chloride, vinylidene chloride,and vinylidene bromide; alkyl ester monomers of maleic and fumaric acidsuch as dimethyl maleate, and diethyl maleate; vinyl alkyl ethermonomers such as vinyl methyl ether, vinyl ethyl ether, vinyl isobutylether, and 2-chloroethyl vinyl ether; vinyl pyridine monomers; N-vinylcarbazole monomers, and N-vinyl pyrrolidine monomers.

The olefinic polymers may also contain a metallic salt form of apolyolefin, or a blend thereof, which contains free carboxylic acidgroups. Illustrative of the metals which can be used to provide thesalts of said carboxylic acid polymers are the one, two and threevalence metals such as sodium, lithium, potassium, calcium, magnesium,aluminum, barium, zinc, zirconium, beryllium, iron, nickel and cobalt.

The olefinic polymers may also include blends of these polyolefins withother polyolefins, or multi-layered structures of two or more of thesame or different polyolefins. In addition, they may containconventional adjuvants such as antioxidants, light stabilizers, acidneutralizers, fillers, antiblocking agents, pigments, primers and otheradhesion promoting agents.

Preferred olefinic polymers include homopolymers and copolymers ofethylene with alpha-olefins as well as copolymers of ethylene and vinylacetate. Representative materials of the latter include Elvax 150, 3170,650 and 750 available from E.I. du Pont de Nemours and Company.

Optionally, one or more layers in a multilayer article of the inventionmay also include known adjuvants such as antioxidants, lightstabilizers, conductive materials, carbon black, graphite, fillers,lubricants, pigments, plasticizers, processing aids, stabilizers, andthe like including combinations of such materials. In addition,metallized coatings and reinforcing materials also may be used in theinvention. These include, e.g., polymeric or fiberglass scrim that canbe bonded, woven or non-woven. Such a material optionally may be used asa separate layer or included within a layer in a multi-layer embodimentof the present invention.

To be most useful, the multilayer articles of the present inventionshould not delaminate during use. That is, the adhesive bond strengthbetween the different layers of the multi-layer article should besufficiently strong and stable so as to prevent the different layersfrom separating on exposure to, for example, moisture, heat, cold, wind,chemicals and or other environmental exposure. The adhesion may berequired between non-fluoropolymer layers or adjacent the fluoropolymerlayer. Various methods of increasing interlayer adhesion in all casesare generally known by those of skill in the art. The article of theinvention may also include a bonding interface or agent between saidouter and intermediate layers.

A variety of methods have been employed to bond polymeric materialscomprising a fluoropolymer to substantially non-fluorinated polymericmaterials. For example, the layers can be adhesively bonded together bya layer of adhesive material between the two layers. Alternatively,surface treatment of one or both of the layers, used independently or inconjunction with adhesive materials, has been used to bond the two typesof materials together. For example, layers comprising a fluoropolymerhave been treated with a charged gaseous atmosphere followed bylamination with a layer of a non-fluorinated polymer. As anotherapproach, “tie-layers” have been used to bond a fluoropolymer materialto a layer of material comprising a substantially non-fluorinatedpolymer.

One specific surface treatment of a fluoropolymer for improving adhesionis disclosed in U.S. Pat. No. 6,630,047, herein incorporated byreference in its entirety. The specific surface treatment involves theuse of actinic radiation, such as ultraviolet radiation in combinationwith a light-absorbing compound and an electron donor.

In a preferred embodiment, one such tie layer method for improvinginterlayer adhesion with the fluoropolymer comprises blending a base andan aromatic material such as a catechol novolak resin, a catechol cresolnovolak resin, a polyhydroxy aromatic resin (optionally with a phasetransfer catalyst) with the fluoropolymer and then applying to eitherlayer prior to bonding. Alternatively, this composition may be used asthe fluoropolymer layer without separate tie layer as disclosed in U.S.Published Application No. 2005/0080210 A1, herein incorporated byreference in its entirety.

Another tie layer method for bonding fluoropolymers is the use of acombination of a base, a crown ether and a non-fluoropolymer. Thismethod is disclosed in U.S. Pat. No. 6,767,948, herein incorporated byreference in its entirety.

Another method that may be used as a tie layer or as a primer forbonding fluoropolymers involves the use of an amino substitutedorganosilane. The method is fully disclosed in U.S. Pat. No. 6,753,087,herein incorporated by reference in its entirety. The organosilane mayoptionally be blended with a functionalized polymer.

Adhesion between non-fluoropolymer layers may also be accomplished in avariety of ways including the application of anhydride or acid modifiedpolyolefins, the application of silane primers, utilization of electronbeam radiation, utilization of ultraviolet light and heat, orcombinations thereof.

In a preferred embodiment, the intermediate layer and the second outerlayer may be combined such as those commercially available as 3M™Scotchpak™ Heat Sealable Polyester Films which include PET filmscombined with olefinic polymers such as polyester and ethylene vinylacetate.

Those of ordinary skill in the art are capable of matching theappropriate the conventional bonding techniques to the selectedmultilayer materials to achieve the desired level of interlayeradhesion.

The multi-layer articles of the invention can be prepared by severaldifferent methods. For instance, one process for preparing a multilayerarticle featuring a fluoropolymer layer of the present inventioninvolves extruding one layer through a die to form a length of film. Asecond extruder supplies a die to coat another layer of molten polymeronto a surface of the first film. Additional layers can be added throughsimilar means. Alternatively, the polymeric resins of two or moresubstituent layers may be co-extruded through a multi-manifold die toyield an intermediate or final product.

Those skilled in the art of coating technology are capable of selectingprocess equipment and processing conditions to address selectedmaterials and thereby produce the desired multilayer film.

Following the extrusion operations, the multi-layer article may becooled, e.g., by immersion in a cooling bath. This process can be usedto form multilayer sheets of the invention. In addition, the layers arepreferably pressed together, such as through a nip or platen or otherknown means. Generally, increasing the time, temperature, and/orpressure can improve interlayer adhesion. The conditions for bonding anytwo layers can be optimized through routine experimentation.

Yet another useful method is to pre-form the individual film layers andthen contact them in a process such as thermal lamination in order toform a finished article of the invention.

The inter-layer adhesion promoting agents, if required, can be appliedeither sequentially, simultaneously or in-situ with any of the beforedescribed processes.

The intermediate layer, prior to application of the outer layers, shouldhave a shrinkage rate of less than 1% at 150° C. when held for about 15minutes, as previously indicated. In that regard, it may be necessary topre-shrink the intermediate layer before the application of the otherouter layers. Even then so, care must be taken with the addition of theouter layers such that inner layer is not overly tensioned or strainedwhich can reintroduce shrinkage into the overall construction.Pre-shrinking of the film after the addition of other layers can becomeexceedingly difficult especially if one or more of the additional outerlayers has a softening or melting point that is within the temperaturerange required to pre-shrink the intermediate layer.

The thickness of the individual layers within the multilayer film can bevaried and tailored per the end-use application requirements. In generalthough, the outer layer of fluoropolymer will be from about 0.5 mils to5 mils, preferably 1 to 2 mils thick; the intermediate layer will befrom about 1 to 10 mils, preferable 2 to 4 mils; and the outerpolyolefin layer will be from 1 to 20 mils or greater, preferable it is10 mils or greater. The thickness of the overall construction istypically 15 mils or greater, and in a preferred embodiment, thethickness of the outer polyolefin layer is as thick, preferably twice asthick, or greater than the combined thickness of the intermediate andfluoropolymer layers.

The multilayer film of the present invention is suitable for various enduse applications. For example, the film may be utilized as a backinglayer on solar cells structures. The use of the multilayer film in thismanner results in a low cost, conformable, readily applied backing.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand principles of this invention, and it should be understood that thisinvention is not to be unduly limited to the illustrative embodimentsset forth hereinabove. All publications and patents are hereinincorporated by reference to the same extent as if each individualpublication or patent was specifically and individually indicated to beincorporated by reference.

1. A multilayer film comprising: a first outer layer of asemi-crystalline fluoropolymer with a tensile modulus of less than100,000 psi; an intermediate layer of polyethylene terephthalate havingat least two planar directions and a shrinkage of less than 1% in eachplanar direction when held for 15 minutes at 150° C. measured inaccordance with ASTM 2305-02; and a second outer layer of an olefinicpolymer of the formula CH₂═CHR″, wherein R″ is hydrogen or a C₁₋₁₈alkyl, and wherein the first outer layer and the second outer layer arebonded to opposing sides of the intermediate layer.
 2. A multilayer filmof claim 1 wherein the first outer layer comprises interpolymerizedunits of tetrafluoroethylene, hexafluoropropylene and vinylidenefluoride.
 3. A multilayer film of claim 1 wherein the first outer layercomprises interpolymerized units of tetrafluoroethylene and ethylene. 4.The multilayer film of claim 1 further comprising applying a tie layerbetween the first outer layer and the intermediate layer, the secondouter layer and intermediate layer, or both.
 5. A multilayer film ofclaim 1 wherein the second outer layer comprises interpolymerized unitsof ethylene vinyl actetate.
 6. A multilayer film of claim 1 wherein thethickness of the second outer layer is greater than or about twice thecombined thickness of the first outer layer and the intermediate layer.7. A multilayer film of claim 1 wherein the thickness of the multilayerfilm is greater than 15 mils.
 8. A multilayer film of claim 1 whereinthe fluoropolymer has a tensile modulus of less than 80,000 psi.
 9. Amultilayer film of claim 1 wherein the shrinkage of the multilayer filmis less than 1% at 150° C. when held for 15 minutes measured inaccordance with ASTM 2305-02.
 10. An article comprising the multilayerfilm of claim 1 applied to a substrate.
 11. The article of claim 10,wherein the substrate is a solar cell.
 12. The article of claim 11,wherein the substrate is a backing layer on the solar cell.